1. Field of the Invention
The present invention relates to an exhaust gas recirculation control device for an engine with a dual cooling system.
2. Description of the Related Art
An exhaust gas recirculation (EGR) control device for an engine, in which a part of an exhaust gas of the engine, as an EGR gas, is recirculated to the engine to improve the exhaust emission, is well known. However, if the supply of the EGR gas is performed when the temperature of the cylinder wall is low, the sulfur fraction in the EGR gas may reach the cylinder wall at the low temperature and be converted to sulfuric acid, and thereby the cylinder wall or the piston ring may be corroded by such sulfuric acid.
To solve this problem, an EGR control device in which the supply of the EGR gas is stopped when the cooling water temperature is lower than a predetermined temperature is also well known, and control is performed when the temperature of the cooling water of the engine is higher than the predetermined temperature.
On the other hand, Japanese Unexamined Patent Publication No. 57-159916 discloses a dual cooling system for an engine, in which the cooling system is provided with a head-side cooling passage formed in the cylinder head, and a block-side cooling passage formed in the cylinder block, and in which the cooling water circulation through the block-side passage can be stopped without stopping the cooling water circulation through the head-side passage. In the cooling system, when the engine is, for example, warming-up, the cooling water circulation through the block-side passage is stopped while the cooling water circulation through the head-side passage is performed, to thereby ensure good cooling of the cylinder head and increase the temperature of the cylinder block, i.e., the cylinder bore wall, rapidly to thereby shorten the time required for warming-up.
On the other hand, the above-mentioned EGR control device may be applied to such an engine with the dual cooling system to stop the supply of the EGR gas when, for example, the temperature of the cooling water circulating through the head-side passage is lower than a predetermined temperature, and to perform the supply of the EGR gas when the temperature of the cooling water is higher than the predetermined temperature. In this case, the predetermined temperature may be set to a temperature of the cooling water above which the supply of the EGR gas is performable with cooling water circulation through both of the head-side and block-side passages being performed. However, in the case where the temperature of, for example, the cooling water circulating through the head-side passage rises, if the cooling water circulation through the block-side passage is stopped while the cooling water circulation through the head-side passage is performed, the temperature of the cooling water circulating through the head-side passage is still lower than the predetermined temperature, even after the temperature of the cylinder bore wall becomes higher than a temperature above which the supply of the EGR gas is performable. Thus, the supply of the EGR gas is not performed during a period from when the temperature of the cylinder bore wall reaches the temperature above which the supply of the EGR gas is performable until the cooling water temperature is higher than the predetermined temperature, and therefore a problem arises that the improvement of the exhaust emission by the EGR gas is not ensured during this period.
Also, in the case where the temperature of the cooling water circulating through the head-side passage drops, if the cooling water circulation through the block-side passage is stopped while the cooling water circulation through the head-side passage is performed, the temperature of the cooling water circulating through the head-side passage becomes lower than the predetermined temperature, even though the temperature of the cylinder bore wall is still higher than the temperature above which the supply of the EGR gas is performable. Thus, also in this case, the improvement of the exhaust emission by the EGR gas is not ensured.